To the Editor Yuan et al1 perform an intriguing and instructive study demonstrating a link between cesarean delivery and obesity. The study size is large and the methods robust. However, the authors overlook an important confounder, namely, birth interval between children. The evidence from meta-analyses and very large studies is almost incontrovertible that mothers who have vaginal deliveries have a shorter interval between this delivery and the birth of their next child when compared with mothers who have cesarean delivery.2 The former are also are more likely to have subsequent children even after adjustment for confounders.2 The reason for this remains unclear but is likely related to parental choice.2 There is very strong evidence that a shorter birth interval between siblings is associated with a decreased risk of obesity.3,4 Consonant with this finding is the observation that only children and last-born children are at increased risk of obesity.3,4 In summary, evidence suggests that children for whom there is a short interval until the arrival of the subsequent sibling are less likely to be obese.3,4 Children born by cesarean delivery tend to have greater birth spacing until the next child and hence may be more likely to be obese for this reason.